1. Field of the Invention
This invention relates to an electromagnetic stirrer attached to a continuous casting machine for making the magnetic field act on non-solidified molten steel existing in an ingot cast by the continuous casting machine to progress through the equi-axes crystal zone of the ingot to prevent segregation or the like for eliminating internal defects of the ingot.
2. Description of the Prior Art
Prior art electromagnetic stirrers attached to continuous casting machines act on the non-solidified ingot, i.e., the ingot containing therein non-solidified molten steel, subject the molten steel to a static magnetic field or the travelling magnetic field so that a current induced at the non-solidified portion in the ingot, or a current flowing directly therein, and said magnetic field generate a Lorentz force, by which the non-solidified molten steel is stirred so that a stirring flow breaks crystals during the solidifying process, or a molten steel temperature is equalized to increase the equi-axes crystal zone, thereby preventing macro segregation and eliminating internal defects
Such electromagnetic stirrers are of various types according to the size and construction of mold. For example, for a tubular type mold of small section used for a billet continuous casting machine, an electromagnetic stirrer of a rotating field type and of the same construction as a stator of an induction motor is used. However, for a casting machine, such as a bloom continuous casting machine or a slab continuous casting machine, using a built-up mold for an ingot of large section, the electromagnetic stirrer of an induction-motor-type and surrounding the ingot is to large to be practical.
Hence, a stirrer of, a linear induction motor type installed only at the long sides of a mold has been used.
FIG. 1 is a perspective view, exemplary of the abovementioned stirrer, in which electromagnetic stirrers 10 are assembled with the long sides 2 of a mold 1 (one stirrer only is shown). FIG. 2 is a schematic sectional view taken on the line II--II in FIG. 1, showing an arrangement of a core 20 and coils 40, 41, 42, 43, 44, 45. Into slots 30, 31, 32, 33, 34, 35 and 36 formed at the surface of core 20 opposite to an ingot are fitted six flat coils 40, 41 . . . 44, 45 each having its major diameter horizontal and perpendicular to the axes of slots 30 through 36; the coils 40 to 45 being fitted in longitudinal arrangement and axially juxtaposed in parallel to the widthwise direction, and being arranged downwardly in the order of phases of V, W and U (the order of three phase: U, V and W) in 2 sets, the adjacent coils being contained in the same slots 31, 32, 33 and 34. The current flowing directions are assumed in the order of V, -V, -W, W, U and -U from above.
This coil arrangement provides two sets of coils or a distance between two electrodes only, which is defined on the basis of a design idea based on an infinite length linear induction motor, whose travelling wave current distribution I is given in the following equation (1): ##EQU1## where k: .pi./.tau.
.tau.: a distance between poles PA1 .omega.: 2.pi.f PA1 f: Frequency of applied AC current PA1 I.sub.o : Maximum value of travelling wave current PA1 t: Time PA1 x: Distance from the center of apparatus.
FIG. 3 shows the current components of Equation (1) for seven slots 30 through 36 at the core 20, which each are divided into two perpendicular components assuming that the phase W is 0.degree.; FIG. 3(a) showing the W-phase component and FIG. 3(b) the component perpendicular thereto.
In such a coil arrangement and energization, the magnetic flux, for example a U-phase current, as shown by the broken line in FIG. 2, perforates magnetic poles (core portions) formed between the slots 32 and 33 and the slots 35 and 36 containing the U-phase coils 42 and 45. Accordingly, currents flowing in U-phase coils 42 and 45 are restricted by the saturation magnetic flux density defined by a sectional area or a width c, thereby creating a problem in that an electromagnetic force or a molten steel stirring force is not sufficiently obtained.
In a case of using a bloom continuous casting machine for an ingot of sectional area of 300.times.400 mm.sup.2 ; assuming that a mold thickness is 30 mm, a length (2.tau.) of a linear induction motor is 500 mm or less, and a distance between the front of linear induction motor and the molten steel is 120 mm; an electromagnetic force of 5000 N/m.sup.3 is required to sufficiently stir the molten steel. However, only 2000 N/m.sup.3 is obtained from computation of the electromagnetic field using the finite element method and the magnetic field measurement by a 1/10 model.
Hence, it is proposed to use a short pitch winding of length 2.tau. and divided into three equal parts as shown in FIG. 4. In this case, sicne the coil number is a half, the sectional area or width of the magnetic pole becomes twofold. Such construction cannot obtain a sufficient electromagnetic force because it is less affected by the magnetic saturation, but an electromagnetic force of 3000 N/m.sup.3 is obtainable under the aforesaid conditions.